A common chip resistor is made by providing the upper surface of a ceramic substrate with a pair of upper electrodes, a resistive element for bridging the upper electrodes, and a protective layer for covering the resistive element, providing the lower surface of the ceramic substrate with a pair of lower electrodes, and providing both longitudinal end faces of the ceramic substrate with an end-face electrode. The end-face electrode is closely attached to the upper and lower electrodes. Each of these electrodes is covered with a plating layer. When the chip resistor is to be mounted, the lower electrodes are positioned on a wiring pattern of a circuit board, and then the wiring pattern is soldered to the plating layer so that electrical power is distributed to the upper electrodes and resistive element through the end-face electrode.
Meanwhile, the resistive element for the above type of chip resistor is often made of a ruthenium oxide material. For a chip resistor that is used, for instance, for electronic circuit current detection, however, it is necessary that its resistance be not higher than 1Ω. Under such circumstances, a low-resistance chip resistor that uses a resistive element made mainly of copper has long been known (refer, for instance, to Patent Document 1). Copper is a low-resistance material and has a small temperature coefficient of resistance (TCR). Therefore, when the resistive element is made mainly of copper, it is possible to obtain a low-resistance, low-TCR chip resistor having a resistance value setting of not higher than 1Ω.
However, even when a resistive element made of a low-resistance material is positioned on the upper surface of the ceramic substrate, the resistive element is electrically connected to the wiring pattern on the circuit board through the end-face electrode. Therefore, when an attempt is made to lower the resistance of the chip resistor, the inductance of the end-face electrode cannot be ignored. When the chip resistor is mounted on the wiring pattern of the circuit board, power is distributed to the upper electrodes and resistive element through the end-face electrode. However, the end-face electrode is extended from the lower end of the ceramic substrate to the upper end. Therefore, a resistance value that would inhibit the chip resistor from lowering its resistance is unavoidably generated by the end-face electrode.
Under the above circumstances, the inventor has focused its attention on face-down mounting, that is, mounting the resistive element side of the chip resistor on the component side of the circuit board, as a method for lowering the resistance of the chip resistor. When the resistive element and its electrode section are positioned on the lower surface of the ceramic substrate of the chip resistor with the electrode section placed on the wiring pattern of the circuit board, power can be distributed to the resistive element while bypassing the end-face electrode. Therefore, the resistive element could be made mainly, for instance, of a copper-nickel alloy to readily lower the resistance of the chip resistor. The face-down mounting technique described above has long been used for the purpose, for instance, of downsizing the chip resistor (refer, for instance, to Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-Open Publication No. H10-144501 (pages 4 and 5, FIG. 1)
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-58303 (page 2, FIG. 9)